The present invention relates to a rolling device such as a rolling bearing, a direct-acting bearing, a ball screw and a shaft and, more particularly, to a rolling bearing optimal to the application to precision machinery.
Rolling bearings have so far been used in precision machinery and in recent years, they have been used also in machinery relating to computers, such as hard disks, and VTRs. In a rolling bearing of a relatively small size used in these fields, there arises the problem of noise, which has been almost trivial in other uses.
In prior arts, high-carbon chromium steels (JIS-SUJ) or JIS-SUS 440C-class stainless steels have hitherto been used as the material of the rolling bearings. The high-carbon chromium steels (JIS-SUJ) are excellent in quiet (, that is, low noise) but inferior in corrosion resistance, whereas JIS-SUS440C-class stainless steels are excellent in corrosion resistance but inferior in quiet (, that is, noise is in a high level).
On the other hand, the medium-carbon 13%-chromium steel disclosed, for example, in JP-A-61-163244 has been generally used as a steel which has improved the quiet (, that is, the problem of noise) of JIS-SUS440C.
In recent years, however, a very high level of quiet is required in comparison with that of the medium-carbon 13%-chromium as machinery such as a hard disk etc. relating to computers comes to have such a high-capacity design as to have, for example, the number of tracks per inch of not less than 25000, and as the machinery comes to have a high-speed design.
In the rolling bearing disclosed in JP-A-61-163244, the quiet and corrosion resistance were able to be improved, in comparison with the conventional high-carbon chromium steels (JIS-SUJ) and the stainless steel of JIS-SUS440C, by specifying the chemical compositions and the size and amount of eutectic carbides. However, this rolling bearing has not necessarily met the level of higher quiet being required because of recent dramatic enlargement in the environment in which this rolling bearing is used.
An object of the invention is to provide a rolling bearing indispensable for the enhancement of the performances of VTRs, machinery relating to computers, and precision instruments, which rolling bearing has remarkably-improved quiet (, that is, remarkably low noise) even at a rotational speed not less than 10000 rpm and which rolling bearing has at the same time a corrosion resistance equivalent to that of the conventional rolling bearings.
The present inventors have researched the effect of carbides and hardness affecting the quiet and have found that the quiet can be greatly improved by adopting the following essential features. The inventors have also found a chemical composition desirable for achieving the features and have further established the details such as a manufacturing method such as the method of a heat treatment, whereby the invention is achieved.
According to the first aspect of the invention, there is provided a rolling bearing comprising an outer race, an inner race, and a plurality of rolling elements interposed between the outer race and the inner race, at least one of the outer race, the inner race and the rolling elements being made of a steel consisting essentially, by mass, of 0.40 to 0.60% C, not more than 0.5% Si, not more than 0.5% Mn, not less than 8.0% but less than 10.0% Cr, and the balance Fe and incidental impurities, said steel having a hardness not less than 740 HV, carbides contained in said steel having a long size not more than 1.2 xcexcm, and an amount of said carbides being not more than 3.5% in area %. Preferably, the carbides contained in this steel have a ratio of (a short size)/(a long size) of 0.9 to 1.0 and an amount not less than 0.5% in area %.
In the steel, it is preferred that the steel consists essentially, by mass, of 0.40 to 0.60% C, 0.1 to 0.5% Si, 0.1 to 0.5% Mn, not less than 8.0% but less than 10.0% Cr, and the balance Fe and incidental impurities. It is also preferred that the steel further contains, by mass, at least one kind selected from the group consisting of W and Mo the total amount of which at least one kind is in the range of 0.1 to 2.0% in terms of (xc2xdW+Mo), and/or at least one kinds selected from the group consisting of Nb and V the total amount of which at least one kind is in the range of 0.05 to 0.50% in terms of (xc2xdNb+V). In addition, it is further preferred that the steel contain, by mass, less than 0.05% N, not more than 0.01% Al, and not more than 50 ppm of O (oxygen).
According to the second aspect of the invention there is provided a method of making a rolling bearing comprising an outer race, an inner race, and a plurality of rolling elements interposed between the outer race and the inner race, the method comprising the steps of: preparing a steel material consisting essentially, by mass, of 0.40 to 0.60% C, not more than 0.5% Si, not more than 0.5% Mn, not less than 8.0% but less than 10.0% Cr, and the balance Fe and incidental impurities; and heat-treating the steel by performing a hardening treatment at a hardening temperature not less than 1000xc2x0 C., a sub-zero treatment at a sub-zero treatment temperature not more than xe2x88x9270xc2x0 C., and a tempering at a tempering temperature of 150 to 200xc2x0 C., so that at least one of the outer race, the inner race and the rolling elements is made. For the steel material, as a matter of course, it is possible to use also the preferred chemical compositions disclosed regarding the preferred steels. Further, it is preferred that the steel material be produced by a remelting process or a continuous casting process and that, more specifically, the steel material be obtained by performing the cold drawing thereof with a reduction of area of not less than 15%.